172 SOIL CONDITIONS AND PLANT GROWTH 



The compound particles can be reformed by careful cultiva- 

 tion and by adequate additions of organic matter and calcium 

 carbonate, but the process may take years, and it cannot be 

 hastened until it is better understood. 



In the preceding pages we have shown how many of the 

 important soil properties are due to colloids. The formation 

 of these compound particles, the absorption of soluble manures, 

 the retention of water (in part), the swelling of the soil when 

 wet and its shrinkage when dry, are all colloidal phenomena. 

 If we regard the mineral particles as the skeleton of the soil 

 we must look upon the colloids as clothing it in many of its 

 essential attributes. How the colloids are arranged in the 

 soil is not known, but the simplest view, and one in accord- 

 ance with all the facts, is that the mineral particles, especially 

 the fine silicate particles, are coated l with a colloidal complex 

 containing silica, alumina, ferric oxide, alkaline bases, and phos- 

 phoric acid derived from the weathering of the rock material 

 and the so-called humus. These various components are not 

 in true chemical combination, but in a state of absorption, or 

 solid solution. The complex is decomposable by changes in 

 temperature, concentration of the soil solution, etc., but it de- 

 composes continuously and not in the per saltern manner of 

 ordinary chemical reactions. It can interact with various solu- 

 tions, absorbing certain substances as a whole e.g. organic 

 dye-stuffs or simply giving up to the solution an amount of 

 base equivalent to what it has absorbed. 



A wholly different conception of the constitution of the 

 soil has been put forward by Whitney of the Bureau of Soils, 

 United States Department of Agriculture, Washington (304-6). 

 Soil particles are supposed to arise by disintegration and to 

 consist of the original minerals of which the rock was com- 

 posed ; little importance is attached to the weathered silicates 

 that play so large a part in the view just set out. Colloidal 

 properties and the special clay properties begin to appear when 

 the disintegration has gone so far that the particles become 

 very minute : these properties are not associated with any par- 

 1 See also 8ga and b. 



